The state-of-charge of a battery may be defined as the amount of electrical energy stored in a battery at a given time, expressed as a percentage of the energy when the battery is fully charged. A need for accurate estimation of this state of charge of a battery power supply exists for many applications, for example in motor vehicles such as electric vehicles and hybrid electric vehicles. Methods known in the art for determining the state of charge of batteries in vehicles may use specific gravity measurements, time-integration of current, voltage measurements, or combinations thereof.
For example, the state of charge may be determined by current integration, which also may be referred to as Coulomb counting, charge counting or Ampere-hour measurement. In current integration methods, the discharge current is continuously measured and integrated over time to obtain an estimate of total expended charge from a known initial amount of charge. However, it may be required to take changes in battery capacity as a function of current discharge history into account to maintain accurate state of charge estimates over the battery lifetime.
Furthermore, the state of charge estimated by current integration may be periodically calibrated by measuring the off-load voltage of the idle battery, e.g. after an extended period of inactivity of the vehicle. A method of this type is, for example, described in German Patent No. DE 35 20 985. Although such methods may be easy to implement, estimation errors may accumulate over long operating phases of the vehicles, having relatively short or only few rest periods, because the state of charge may only seldom be re-calibrated or corrected via a measurement of the off-load voltage.
Other known methods may be based on battery models for determining the off-load voltage for a battery under load, and hence the state of charge derivable from this off-load voltage, via adaptation of a model to the actual battery using observed variables, such as battery voltage, battery current, and/or battery temperature. However, such model-based methods may be relatively complex.
Voltage analysis methods may measure the terminal voltage and determine state-of-charge in various manners. With respect to such analyzers, polarization effects, e.g. as caused by charging or fast discharging, can adversely change the indication of the analyzer meters, and most such analyzers tend to be only uni-directional, for example, may work accurately on discharge only, with no provision for aging effects.